Unfortunately, these Toy Spas do not give you the ability to make repairs to their components! Including the heater, so no ability to replace an element.
For a failed heater, it appears that you have to replace the complete control center. Because parts are extremely to impossible, to obtain.
Contact them, and do some squeaking, maybe they will oil that squeak?
Look here... www.intexcorp.com/support

It is more than likely your heating element. Not sure what style they use but if you send me a pic of your equipment, spa pack w/ the cover removed, I can walk you thru how to test it.

If you have a good grasp of electricity, find the heating element and with the power off remove the leads. Try to plug it back in, reset the gfci, and give it a minute. If it doens't trip and everything runs fine replace the heating element. With most spa packs it is possible to replace only the element, some like watkins you have to replace the whole heater. If the spa still trips with the heater disconnected start unplugging your equipment starting with the ozone, then blower, then pumps, one at a time until your braker stays on.

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DRAINING THE SPA:
1. Unplug power pack from electrical receptacle before draining spa.
2. Put the rubber plug into the outlet in the inner wall of the spa.
3. Open the outlet in the outer wall of the spa. Screw one end of inflation hose into it.
4. Remove the rubber plug from the outlet in the inner wall of the spa.This will allow water to flow out of the spa. 5.When water is completely drained, press BUBBLE and run for 2-3 minutes to drain the water in the air pipeline. 6.When drainage is finished, remove the inflation hose and replace cap on outside of spa. Store rubber plug for future use.

SPA STORAGE
Make sure all water is drained from the spa and control box.This is essential to extend the life of the spa.We recommend the use of a wet/dry vacuum to blow out or suck up water from the spa pipes and pump lines. Remove the filter cartridge. It is recommended you store the spa in its original package in a warm, dry place.
Note: Plastic becomes brittle and susceptible to breakage when exposed to subzero temperatures. Freezing can severely damage the spa. Improper winterization of your spa may void your warranty.
http://img1.wfrcdn.com/docresources/17478/11/119999.pdf

other than the heater having a fault inside of it (common problem) the only thing that a user can do is to make sure it was proper power connection (outlet is live) and make sure it has good flow thru the heater , as they do have a flow switch that prevents heater from coming on if water flow isn't enough(safety sensor)